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Glaucus atlanticus and its close relative, Glaucilla marginata, live in close association with what Sir Alistair Hardy described many years ago as "The Blue Fleet" - the siphonophores such as Physalia, Velella, Porpita and the other associated animals including the "Violet snails" of the genus Janthina. All these animals float on the surface of the ocean being carried by the currents and the winds. Most of us are only aware of their existence when days of onshore winds blow great fleets of them on to the beaches, causing pain and angst for swimmers.

Both species spend their life floating upside down in the water, partially bouyed by a gas bubble in their stomachs.

The two nudibranchs feed almost exclusively on Physalia, and as Tom Thompson and Isobel Bennett reported some years ago, it appears that they are able to select the most venomous of Physalia's stinging nematocysts for their own use. Like most aeolids, they store the nematocysts in special sacs (cnidosacs) at the tip of their cerata .

There are a number of reports in Australia of kids engaged in "Bluebottle" fights - where they throw stranded Physalia at each other - being badly stung by inadvertently playing with Glaucus and Glaucilla, both of which, by concentrating the most venomous of Physalia's nematocysts, are much more deadly.

Another interesting feature of the two species is their colouration. They both exhibit a textbook example of colour countershading. Their foot and undersides of the cerata, (which because they float upside down is effectively their dorsal surface), is blue or blue and white which helps to camouflage them from predation (sea birds) from above. Their true dorsal surface, which faces down in the water, is silvery grey to effectively camouflage them from fish looking up from below.

References:• Bergh, L.S.R. (1884) Report on the Nudibranchiata dredged by H.M.S. Challenger during the years 1873-1876. Report of the Scientific Results of the Voyage of H.M.S. Challenger during the years 1873-76, ... Zoology, 10(26): 1-154, pls. 1-14. [Glaucus: p.10-16]• Forster, G. (1777) A voyage round the world in His Britannic Majesty's sloop, Resolution, commanded by Capt. James Cook, during the years 1772, 3, 4, and 5 by George Forster. Vol 1: p49.• Thompson, T.E. & McFarlane, I.D., 1967. Observations on a collection of Glaucus from the Gulf of Aden with a critical review of published records of Glaucidae (Gastropoda, Opisthobranchia). Proceedings of of the Linnean Society, 178: 10-123. • Thompson,TE. & Bennett,I., 1969. Physalia nematocysts: Utilised by mollusks for defense. Science, 166: 1532-1533.• Thompson,TE. & Bennett,I., 1970. Observations on Australian Glaucidae (Mollusca: Opisthobranchia). Zoological Journal of the Linnean Society, 49: 187-197.